In many applications, instead of using a single power converter, a number of power converters with a lower power rating are employed to bring about performance improvements and/or reduce the cost. For example, paralleling of power converters is a widely used approach in today's high-efficiency, high-power density applications since it makes possible to implement redundancy, as well as to improve partial-load efficiency by employing power management. Similarly, in applications with a relatively high input voltage, instead of a single converter, a number of converters are used by connecting their inputs in series and their outputs in parallel. Series connection of converters' inputs makes possible to use converters designed with lower-voltage-rated components, which are typically more efficient and less expensive than their high-voltage-rated counterparts.
As an example, FIG. 1 shows a prior art method of two converters with their inputs connected in series and outputs in parallel. In applications where a front end provides voltage to downstream dc/dc converters, for example in ac/dc applications, the inputs of the converters are directly coupled to energy-storage capacitors of the front end, as illustrated in FIG. 1. In the connection of the converters in FIG. 1, the balance of the input voltages i.e., the balance of the capacitor voltages, can only be maintained if the converters are identical and capacitors C1 and C2 have same characteristics. Otherwise, the input voltages of the two converters will be different depending on the mismatching of the two converters and capacitors.
Generally, an input voltage imbalance that does not produce excessive stresses on the converter's components can be tolerated. However, to prevent the input-voltage imbalance from exceeding a permissible range, a voltage-balancing control must be implemented in the circuit in FIG. 1.
FIG. 2 shows a prior art voltage balancing method. The simplest voltage balancing method is to connect resistor R across capacitors C1 and C2, i.e., across the inputs of each converter, as shown in FIG. 2. While this method is effective in balancing capacitors voltages due to a typical mismatching of the capacitor impedances, its effectiveness in balancing voltages due to a mismatching of the converters is very limited. Namely, to balance the input voltages of mismatched converters, the difference of the input currents of the converters ΔIIN=IIN1−IIN2 must be taken by the resistors R. As this difference is increasing as the mismatching of the converters increases, the value of resistors R must be decreased. A decreased value of resistance leads to an increased power loss on these resistors which adversely affects the conversion efficiency. As a result the resistor voltage-balancing method shown in FIG. 2 is only suitable for balancing of the capacitor impedances and a very small input-current mismatching of the converters.
FIG. 3 shows another prior art input-voltage balancing method. This active input-voltage balancing method is implemented with a totem-pole switch configuration operated at a 50% duty cycle and an inductor connected between the mid points of the switches and capacitors. Since with 50% duty cycle, the mid voltage of switches VS is exactly one-half of the total capacitor voltage (input voltage) VIN, i.e., VS=VIN/2, the capacitor mid-point voltage VM is also equal to VIN/2 since average inductor voltage is zero. As a result, the voltage balancing circuit ensures excellent balancing and can handle a relatively large mismatching of converters' input currents effectively. The major drawback of the circuit is that it requires additional power components which decreases the efficiency and power density and increases the cost.
FIG. 4 shows yet another prior art input-voltage balancing method. In this method, voltage balancing is implemented solely at the control level, i.e., without addition of any components in the power circuit. As a result, this method offers voltage balancing without adversely affecting, the efficiency, power density, or cost. The approach in FIG. 4 achieves input-voltage balancing by directly regulating input voltage of the converters. Specifically, in this method a controller is used to regulate the input and output voltage of each converter. As illustrated in FIG. 4, for each converter the input and output voltage are sensed and the sensed voltages are brought to the controller. Because the control variable in each of the converter has to simultaneously regulate the input and the output voltage, a simultaneous tight regulation of both is not possible. Since in a majority of applications the output has to be tightly regulated, the input voltage balancing of this method is relatively poor. Since in this method the input voltages are not tightly matched, this method also requires load current-sharing control to ensure that each converter delivers approximately the same amount of the load currents.
Therefore, there exists a need for input-voltage balancing of power converters that have inputs connected in series and outputs connected in parallel without the limitations of prior-art techniques.